1. Field of the Invention
The present invention pertains to a method of protecting surfaces of semiconductor devices and micro-electro-mechanical systems (MEMS) during fabrication and handling.
2. Brief Description of the Background Art
The presentation of information in this Background Art section is not an admission that any of the information is prior art to the present invention. The purpose of the information provided is to aid one skilled in the art in understanding of the invention.
Both integrated circuit (IC) device fabrication and micro-electro-mechanical systems (MEMS) fabrication make use of layers or coatings of material which are deposited on a substrate for various purposes. In some instances, the layers are deposited on a substrate and then are subsequently removed, such as when the layer is used to create a patterned masking material and then is subsequently removed after the pattern is transferred to an underlying layer. In other instances, the coating layers are deposited to perform a function in a device or system and remain as part of the fabricated device. There are numerous methods for depositing a thin film layer or a coating. In sputter deposition, a plasma is used to sputter atoms from a target material (commonly a metal), and the sputtered atoms deposit on the substrate. In chemical vapor deposition, activated (e.g. by means of plasma, radiation, or temperature, or a combination thereof) species react either in a vapor phase (with subsequent deposition of the reacted product on the substrate) or react on the substrate surface to produce a reacted product on the substrate. In evaporative deposition, evaporated material condenses on a substrate to form a layer. In spin-on, spray-on, or dip-on deposition, a coating material is applied, typically from a solvent solution of the coating material, and the solvent is subsequently evaporated to leave the coating material on the substrate.
Semiconductor and MEMS structures are often relatively fragile in nature due to their size and composition. During fabrication and handling of the devices, prior to completion of fabrication, a number of hazards may arise which cause contamination, mechanical separation, chemical corrosion, and mechanical scratching or marring of device surfaces, for example and not by way of limitation. Application of a protective coating to protect fragile surfaces at times during fabrication could be particularly beneficial. However, the protective coating should not interfere with subsequent processing of the semiconductor or MEMS device. This is complicated by the fact that it is particularly difficult to apply a coating over only a portion of the semiconductor or MEMS device surface, and a coating which may protect one portion of the device may interfere with subsequent fabrication of other portions of the device. In some instances, it is necessary to remove the protective coating from even the surface it was initially designed to protect, prior to proceeding further with fabrication.
Chlorosilanes have been widely used as a functional attachment group on organic molecules, where it is desired to bond the organic molecule to various surfaces. Typically the organic molecule containing the chlorosilane end group is either deposited from a liquid or from a vapor upon the substrate. When hydroxyl groups are present on the surface upon which a chlorosilane is deposited, the chlorosilane can be hydrolyzed and bonded to that surface. If the number of hydroxyl groups present on the substrate surface are inadequate to provide sufficient bonding sites to ensure a continuous coating of the surface by organic molecules presenting the chlorosilane end group, moisture may be added prior to or during deposition of the organic molecule with chlorosilane end group. An oxide layer with active hydroxyl groups which are available to react with functional organic molecules may be generated first on the surface, followed by attachment of the molecule presenting the chlorosilane end group. The other end of the functional organic molecules typically presents a functional group which provides the characteristics desired on the exterior surface of the coated substrate.
The problem with using a chlorosilane as a bonding agent is that many of the MEMS structures and semiconductor structures as a whole contain metals. Both chlorine and hydrochloric acid are particularly good at reacting with metals and tends to attack and corrode any metals which become exposed to the chlorosilane, especially when excessive concentration of moisture is present in the reaction.
An article entitled “Vapor Phase Anti-Stiction Coatings For MEMS”, by W. Robert Ashurst et al., IEEE Transactions On Device Materials Reliability, Vol. 3, No. 4, December 2003, describes the development of vapor-phase anti-stiction processes for use in the fabrication of MEMS. Of the classes of precursors used for vapor deposition of anti-stiction coatings, the following were mentioned, chlorosilanes, amines, alcohols, carboxylic acids, siloxanes, and dimethylaminosilanes. The substrate surface to which the anti-stiction coatings were bonded was a silicon wafer, which is commonly used as a basic starting substrate for MEMS devices.
In the Ashurst et al. article, the specific chlorosilanes which were discussed included the following tri-chloro silanes: octadecyltrichlorosilane (CH3(CH2)17SiCl3) (OTS) which was deposited from a solution; tridecafluoro-1,1,2,2-tetrahydrooctyltirchlorosilane (CF3(CF2)5(CH2)2SiCl3) (FOTS) which was deposited from a vapor; and hepta-decafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane (CF3(CF2)7(CH2)2SiCl3) (FDTS) which was deposited from both a liquid and a vapor. The dichlorosilane which was discussed was dimethyldichlorosilane ((CH3)2SiCl2) (DDMS), which was deposited from both a liquid and a vapor.
In the Ashurst et al. article, the description of the use of alcohols and amines pertained to a silicon substrate which was chlorine-terminated, which was reacted with an amine or an alcohol of the form RNH2 or R—OH. The chlorine-terminated silicon substrate was prepared by exposing an H-terminated Si sample to Cl2 in a vacuum while either heating the substrate to about 80° C. or illuminating it with a tungsten filament. When either R—NH2 is reacted or R—)H is reacted, HCl is eliminated as a by-product. HCl is known to be particularly corrosive to metals.
In the Ashurst et al. article, the description of carboxylic acid based monolayers is limited to the instance when the substrate, structural layer is aluminum where the application is the Digital Micromirror Device or DMD™, which consists of an array of a million or more rotatable aluminum mirrors. About 50 different lubrication schemes were investigated for the DMD™. The most successful lubricants reportedly are perfluorinated n-alkanoic acids (CnF2n-1O2H), which is said to form self-assembled monolayers (SAMs) on aluminum oxide surfaces. Within this class of SAMs, perfluorodecanoic acid, where n=10 was said to be the lubricant of choice in order to minimize the friction coefficient and the possibility of thermal decomposition.
In the Ashurst et al. article, the description of the use of tris-dimethylaminosilanes specifically mentioned (tridecafluoro-1,1,2,2,-tetrahydrooctyl)tris-dimethylamino silane (PF8TAS) and (heptadecafluoro-1,1,2,2,-tetrahydrodecyl)tris-dimethylamino silane (PF10TAS). These precursors are said to not be commercially available, but to be synthesizable from their corresponding trichlorosilanes (FOTS and FDTS, respectively) and dimethylamine. It is mentioned that the aminosilane precursors are extremely sensitive to water, and must be kept rigorously anhydrous. No processing parameters are given. The aminosilane coating PF8TAS is said to have been characterized on Si(100) and on microengines.
An article entitled “Hydrophobic Coatings Using Atomic Layer Deposition and Non-Chlorinated Precursors”, by C. F. Herriman et al., 0-7803-8265-X/04 ©2004 IEEE, describes a method of depositing hydrophobic coatings on MEMS devices using atomic layer deposition) (ALD) and non-chlorinated hydrophobic precursors. First, a thin film of Al2O3 is deposited via ALD and is used as a seed layer to prepare and optimize the MEMS surface for the attachment of the hydrophobic precursor. Subsequently, non-chlorinated alkylsilanes are chemically bonded to the surface hydroxyl groups on the ALD seed layer. The alkyl silane which was described in the application was tridecafluoro-1,1,2,2-tetrahydro-octyl-methyl-bis(dimethylamino) silane (C8F13H4(CH3)Si(N(CH3)2)2).
In another publication of interest, titled “Vapor Deposition of Amino-Functionalized Self-Assembled Monolayers on MEMS”, Proceedings of SPIE Vol. 4980 (2003) SPIE-0277-786×/03, Hankins et al. describe problems with stiction which occur when a device is removed from a liquid phase into ambient air. A method for dealing with stiction, which is termed VSAMS (vapor-deposited self-assembled monolayers), employs supercritical CO2 drying and chemical vapor deposition to address many of the stiction concerns. Films deposited as part of the VSAMS coating deposition scheme make use of amino-functionalized silanes. The advantage of using amino-functionalized silane precursors for VSAMS is said to be related to strength of the bond between the film and an underlying polysilicon surface (as evidenced by the stability of the films made with precursors across “the entire humidity scale”). The authors indicate that there is good evidence that the films formed using perfluorinated chlorosilane precursors are not bonded as deposited directly on the silicon surface of a MEMS substrate, and require an annealing step for the coating to become firmly attached to the substrate. Films formed using perfluorinated aminosilane precursors are said to be bonded as deposited, so that an annealing step is not required. Two perfluorinated aminosilane precursors were evaluated. These were (tridecafluoro-1,1,2,2,-tetrahydrooctyl)tris(dimethylamino)silane and (heptadecafluoro-1,1,2,2,-tetrahydrodecyl)tris(dimethylamino)silane. In particular, the films formed using these precursors were applied to microengines with two orthogonal comb drive arrays to provide lubrication during operation of the microengines.
The use of vapor deposited films formed from fluorinated aminosilane and fluorinated chlorosilane precursors provides a hydrophobic surface on a substrate, and such a hydrophobic surface has been demonstrated to help prevent stiction in MEMS devices.
There are a number of functionalized silane-containing precursor materials and functionalized organic-based precursor materials which can be vapor deposited to form thin films on a variety of different substrates. One skilled in the art must develop a coating system that will provide the results which are required in a particular application. This typically requires extensive research and developmental efforts. In the present instance, the inventors were working to find thin film coatings which could act as a protective coating during processing of semiconductor substrates, or biomedical substrates, or MEMS substrates, where there may be a need to selectively remove a portion of the protective coating or to remove the entire protective coating during a fabrication process without affecting functional portions of the device which is being fabricated.